Figure-9 aluminium rope tensioners from eBay (US $0.62 each). -or-
Nite Ize MiniCamJam Cord Tightener $2.49 for 2 at Cabellas. Home Depot used to carry these too, w/o rope.
Plus a suitable length of fluorescent orange 275 paracord, or similar. Extra lengths of paracord for attaching to bigger trees, etc. with loops on both ends.
No knots needed or ever used at the summit.
Inexpensive, very lightweight, flexible, low bulk, and fast. Works with poles, posts, trees, brush, dead-fall, table/chair legs, trekking poles, etc
Nite Ize Gear Ties (various lengths, bright orange). For attaching a mast to: small poles, branches, bush, fence wire, and used as anchors between/around rocks.
Stuff sacks filled with local: gravel, scree, rocks, etc., are good for anchors.
I also carry a supply of Nite Ize S-Biners, size 1 (stainless steel) and size 0 (stainless or plastic) from Home Depot and other vendors. Great for clipping antenna wire to rope/line, rope-to-rope, small branches, loops on anchors, loops on tent pegs, etc.